1. Field of the Invention
The present invention relates to a variable gain amplifier capable of controlling a gain by a control voltage and more particularly to a variable gain amplifier in which a gain is completely proportional to a control voltage.
2. Description of the Prior Art
In a circuit which amplifies an analog signal with a desired variable gain, for example, it is convenient to employ a variable gain amplifier in which the gain is completely proportional to a control voltage in order to set the gain digitally without difficulty by a command from a computer and the like.
FIG. 11 is a circuit diagram showing a conventional variable gain amplifier of this kind, which is disclosed in the document, Paul R. Gray and Robert G. Meyer "Analysis and Design of Analog Integrated Circuit" published by John Wiley & Sons, Inc., in 1977. The variable gain amplifier uses a well-known double balanced modulator, in which a constant current I.sub.EE is divided into currents I.sub.5 and I.sub.6 by a differential pair of transistors Q.sub.5 and Q.sub.6 in accordance with an input voltage v.sub.in. The current I.sub.5 is divided into currents i.sub.1 and i.sub.2 by a differential pair of transistors Q.sub.1 and Q.sub.2 in accordance with a control voltage V.sub.AGC, and the current I.sub.6 is divided into currents i.sub.3 and i.sub.4 by a differential pair of transistors Q.sub.3 and Q.sub.4 in accordance with the control voltage V.sub.AGC. An output voltage v.sub.out is supplied by the voltage drop generated at resistors R.sub.L.
The gain A.sub.v of the variable gain amplifier is expressed by the following equations: ##EQU1## k: Boltzmann's constant T: absolute temperature
q: electronic charge
FIG. 12 is a graph showing the change in the gain A.sub.v with respect to the control voltage V.sub.AGC. The first quadrant of the graph is a region as a non-inverting amplifier, and the third quadrant thereof is a region as an inverting amplifier.
In the conventional variable gain amplifier as above constructed, the gain A.sub.v is variable in proportion to the control voltage V.sub.AGC in the almost full region of the gain A.sub.v between -1OO% and +100%, as is apparent from the graph of FIG. 12. However. in the regions of the gain A.sub.v around +100% and -100%, the gain A.sub.v does not change in proportion to the control voltage V.sub.AGC. Therefore, when the variable gain amplifier shown in FIG. 11 is used as an amplifier in which the control voltage and the gain are completely proportional to each other, the regions of the gain A.sub.v around +1OO% and -100% are not available.